Federal Election Commission filings show that if a wave crashes on the Republican House majority in November, as many have predicted, Democratic surfers will be on their boards to catch it. Nearly a year out from the election, Democratic candidates have filed in all but 20 House districts held by Republicans. By comparison, Democrats in 80 districts do not have a Republican opponent for their seat.

The Democrats are not just filing to run in districts where Mrs. Clinton performed well. They are also running for conservative seats that were uncontested in 2016 and where Republicans remain heavy favorites, in states like Texas, Arkansas and Nebraska.

Four Democrats — a farmer, a soccer coach, an immigration lawyer and a health care consultant — are on the primary ballot in Illinois’ 16th District, a C-shaped district outside Chicago running from Wisconsin to the Indiana border. The Republican incumbent, Representative Adam Kinzinger, ran unopposed in 2016, when President Trump won the district by 17 points. Mr. Kinzinger is not high on anyone’s endangered list.